A method for engine control for gasoline-direct injection internal combustion engines having NOx-storage catalytic converters is already known from U.S. Pat. No. 6,119,449. The method provides for a modeling of the NOx-storage catalytic converter and a control (open loop and/or closed loop) of the change between storage operation and regeneration operation as well as a catalytic converter diagnosis. An essential element is the computation of the NOx raw mass flow at the input of the catalytic converter from characteristic field data via the input quantities engine rpm, relative fuel mass entry (referred to full load), exhaust-gas recirculation component and desired lambda.
The differences in the intake air temperature, for example, xe2x88x9220xc2x0 C. in a Scandinavian winter and +40xc2x0 C. in the tropical or subtropical summer and in the engine block temperature are not considered in the known computation. The engine block temperature corresponds, for example, during a cold start to the ambient temperature and can increase at full load to the regions of the maximum permissible engine oil temperature.
In a known control for gasoline-direct injection engines, a temperature (Tein) is determined from the intake air temperature and the engine temperature. This temperature is characteristic for the enclosed gas mixture at the start of the compression.
With this background, it is the task of the invention to improve the modeling of the NOx raw mass flow at the input of the catalytic converter.
This task is solved by considering the intake air temperature and the engine oil temperature when modeling.
The intake air temperature and the engine oil temperature are available in modern engine controls as measurement signals.
According to the invention, the NOx mass flow at the catalytic converter input is computed with greater accuracy than previously with the aid of these data and with an NOx emission characteristic field as a function of engine rpm, the relative fuel mass, the exhaust-gas recirculation rate and the desired lambda value. A further increase of the accuracy is made possible in an advantageous embodiment by considering the water vapor content of the intake air.
One embodiment of the invention provides that the NOx mass flow (msnovk) forward of the catalytic converter is computed as the product of a base value (msnovk0) and a temperature-dependent factor (exp(FNOXBAKT*delTemv). The base value (msnovk0) is referred to as a defined normal state.
A further embodiment provides that the temperature-dependent factor is proportional to a change (delTemv) of the combustion temperature as a consequence of changes of an inlet temperature quantity (Tein) which is combined from air temperature and engine temperature.
A further embodiment provides that the computation of the NOx mass flow ahead of the catalytic converter in an internal combustion engine takes place in stratified operation differently than in homogeneous operation. The internal combustion engine can be operated in a first operating mode with a layered mixture distribution in the combustion chamber (stratified operation) and in a second operating mode with a homogeneous mixture distribution in the combustion chamber (homogeneous operation).
According to a further embodiment, a factor for a lambda-dependent NOx formation activation is considered in the homogeneous operation.
A further embodiment provides that an increasing water vapor component of the intake air acts in the computation to reduce the NOx mass flow forward of the catalytic converter.
The invention is also directed to an electronic control arrangement which executes the above-mentioned methods.
The formulation of the present invention is so simply structured that it can be integrated into the engine control without great difficulty and without it being necessary to implement an existing NOx emission characteristic field additionally for deviating intake air and engine temperatures. Stated otherwise, a base characteristic field, which is referred to a normal state and/or exhaust-gas test conditions, can be used further. Deviations of the actual state from the normal state are considered by logically coupling the characteristic field values to corrective values, especially via a multiplicative logic coupling.
In this way, the computation of the NOx mass flow can supply the desired additional information more rapidly and with less use of storage space.